On remand from the Supreme Court, the US Court of Appeals for the Federal Circuit reconsidered the boundaries of the doctrine of assignor estoppel. The Federal Circuit found that the patent assignor was estopped from challenging the validity of an asserted patent because the asserted claim was not materially broader than the specific claims assigned to the patent owner. Hologic, Inc. v. Minerva Surgical, Inc., Case Nos. 2019-2054; -2081 (Fed. Cir. Aug. 11, 2022) (Stoll, Clevenger, Wallach, JJ.)

Csaba Truckai filed a patent application for a device that was designed with a moisture-permeable head to treat abnormal uterine bleeding while avoiding unintended burning or ablation. Truckai assigned the pending patent application to his company, Novacept, which was later acquired by Hologic. Truckai then founded a new company, Minerva Surgical, and developed a new device that used moisture impermeability to avoid the unwanted ablation. Hologic subsequently filed a continuation application to expand the scope of its claims to encompass applicator heads in general, regardless of moisture permeability. The US Patent & Trademark Office issued a patent on the expanded claims in 2015, and Hologic subsequently sued Minerva for patent infringement.

Hologic argued that doctrine of assignor estoppel barred Minerva from challenging the validity of the patent claims. The district court agreed and granted summary judgment of infringement. On appeal, the Federal Circuit affirmed the summary judgment of no invalidity. The Supreme Court granted certiorari and declined Minerva’s request to discard the doctrine of assignor estoppel but clarified that it comes with limits, holding that “assignor estoppel applies only when an inventor says one thing (explicitly or implicitly) in assigning a patent and the opposite in litigation against the patent’s owner.” The Supreme Court remanded to the Federal Circuit to address whether Hologic’s claim was materially broader than the one Truckai assigned. The Supreme Court explained that if the asserted claim was materially broader than the assigned claim, “then Truckai could not have warranted its validity in making the assignment and without such a prior inconsistent representation, there is no basis for estoppel.”

On remand, the Federal Circuit considered whether Truckai warranted the assigned claim’s validity at the time of assignment and whether the assigned claim was materially broader than the asserted claim.

The Federal Circuit concluded that Truckai had represented that the assigned claim was valid. The Court explained that the assigned claim was initially rejected as being anticipated, but Truckai successfully argued for its allowance. The claim was then canceled in response to a restriction requirement, but such cancellation did not speak to the claim’s patentability because an assignee would understand that it could later prosecute the claim’s subject matter under standard patent practice. Therefore, cancelation did not nullify the claim, and it “remained viable for further prosecution.” Additionally, the assignment was not just to the rights to the application, but to the rights to any continuation, continuation-in-part or divisional patent applications not yet filed. When presenting the application, Truckai signed an oath of his implicit good faith belief that the application’s claims were patentable and would result in a valid patent. This representation was affirmed twice more—once when Truckai successfully defended the claim from the examiner’s anticipation rejection and again in the assignment, which contained the warranty that Truckai had “no present knowledge from which [he] could reasonably conclude that these assigned intellectual property rights were invalid or unenforceable.”

The Federal Circuit next determined that the assigned claim was materially broader than the asserted claim. The Court explained in a footnote that it would not define the line between “broader” and “materially broader,” as the parties had agreed previously that if the assigned claim was limited to moisture-permeable devices, then the asserted claim was materially broader and assignor estoppel would not bar Minerva’s invalidity defense. The assigned claim did not have an express moisture-permeability limitation; the plain claim language was broad enough to encompass moisture-impermeable devices. Although Minerva argued that the assigned claim could not cover a moisture-impermeable device, the Court explained that the claim language of the open-ended “comprising” indicated that the claim covered broadly both moisture-permeable and impermeable devices. Using the intrinsic record, the Court explained that moisture permeability was merely a preferable characteristic in the written description, not a mandatory or required one, and it would be improper to restrict the claim to its preferable characteristic. The principle of claim differentiation also supported the conclusion that there was no requirement of a moisture-permeable material, as the other claims in the application expressly recited moisture permeability. In other words, Truckai knew how to draft claims that required moisture permeability and, not including this limitation in the assigned claim, indicated that he did not intend to limit the claim as such.

Because Truckai had warranted the assigned claim’s validity and the assigned claim was materially broader than the asserted claim, Minerva was estopped from challenging the validity of the asserted claim based on doctrine of assignor estoppel.